HCS is readily crystallized under a variety of solution conditions. It has been obtained in different crystal forms from ammonium sulfate, and by dialysis against deionized water. One of these forms offers promise for high resolution X-ray structure analysis. The synthesis of three analogs of the COOH-terminal 51-amino acid fragments were accomplished by the improved procedure of the solid phase method. These analogs are: (Nle 170, Ala 165, 182, 189)-HGH-(150-191), (Nle 170, Ala 165, 182, 189)-HGH-(154-191) and Lys 135, 136, 138, Glu 137, 139, Nle 170, Ala 165, 182, 189)-HGH-(135-191). When the NH2-terminal fragment (Cys(Cam)53)-HGH-(1-134) complemented with inactive synthetic (Lys 135, 136, 138, Glu 137, 139, Nle 170, Ala 165, 182, 189)-HGH-(135-191) or (Nle 170, Ala 165, 182, 189)-HGH-(150-191), the purified recombinants were fully active as revealed by the tibia test. Under the described conditions, HGH and related peptides were added to the cultures of K562 cells in concentrations from 25-500 ng/ml. HGH was a potent stimulant for the proliferation of the human erythroleukemia cells with peak activity of 159 percent of control at 100 ng/ml. Higher concentrations caused a reduced cloning efficiency. The biologically active fragment derived from HGH containing the first 134 amino acids was also effective in this system, but maximally stimulating concentrations (100 mg/ml) has less than half the activity of the intact HGH. At 25 ng/ml activity of the fragment was barely detectable. BGB was inactive when tested over a wide concentration range. The complete HGH dose-response curve for K562 colonies showed a stimulatory effect of 0.1 ng/ml.